1. Field of the Present Description
The present invention relates to the reinforcement of composite laminates and composite laminate bonded joints. In particular, the present invention relates to Z-direction reinforcement of laminated fiber preforms and laminated fiber preform bonded joints.
2. Description of Related Art
An unmanned aerial vehicle (UAV) is defined as a powered, aerial vehicle that has does not carry a human operator, flies autonomously or is piloted remotely, and is expendable or recoverable. When control is exercised by a remote pilot, control may be continuous or episodic. However, autonomous vehicles may follow preprogrammed courses and may or may not have the capacity for rerouting or re-targeting.
Fiber composite laminates are formed by building up multiple layers of composite fibers one upon another. Each layer of fibers is oriented in a specific direction to provide particular properties to the laminate. In a typical laminate, some fibers extend parallel to the longitudinal axis, others extend transverse to the longitudinal axis, and others extend “off-axis” at various angles to the longitudinal axis. By laying up and orienting the fiber layers in certain configurations, the stiffness and other properties of the laminate can be predetermined. A “preform” is a dry fiber composite laminate. Often these preforms are “tackified,” i.e., treated with a material that binds the fibers together, so that the preforms can be more easily handled, shaped, worked, and laid up until the resin is introduced into the preform.
The composite fibers provide strength to the laminate in the plane of the material, but the only material perpendicular to that plane (the Z direction) is the resin. Thus, interlaminar delamination is a common form of failure in fiber composite laminates. Reinforcement of fiber composite laminates in the Z-direction is one way to prevent propagation of delaminations. However, Z-direction reinforcement often creates modifications, alterations, and disruptions to the basic structure of the laminate and generally weakens and softens the laminate. This reduction in strength of the laminate is commonly referred to as “knockdown.” There are several methods of providing Z-direction reinforcement, including Z-pinning, stitching, 3-dimensional weaving, and needling.
Z-pinning is a process by which rigid pins are forced through a prepreg laminate, which is a laminate pre-impregnated with resin. The rigid pins are inserted into a thin piece of foam. The foam is then placed on top of the prepreg. Next, the pins are forced through the composite fibers in the prepreg. An ultrasonic horn is typically used to vibrate the pins through the prepreg.
There are several problems with Z-pinning. Working with prepreg material requires specialized storage and handling, which is expensive and labor intensive. Second, the insertion process breaks some composite fibers. Broken fibers reduce the integrity of the basic laminate. Third, because the prepreg is partially cured with resin, there is less void space between the fibers to accept the pins. This results in undesirable deformation of the fibers.
In stitching, the layers of the fiber preform are mechanically sewn together. The needle punctures through the preform from one side, and the stitching thread is caught by a similar stitching material as it exits on the other side of the preform. In some instances, random mat of chopped fibers are used as layers within the stitched perform laminate. The chopped fibers are not pulled back through the layers of the preform by the needles.
There are several drawbacks to the stitching method. In stitching, the dry composite fibers must be held in tension over platens in large machines. The stitching method requires machine components on both sides of the preform. Furthermore, with the stitching method, the stitching material is tightly woven around the composite fibers, leaving no way to join one laminate to another with Z-direction bondline reinforcement.
In 3-dimensional weaving, Z-direction reinforcement is provided by interweaving reinforcement fibers in the Z direction with the fibers in the X and Y directions. Although this method provides straight fibers in three directions, it is very difficult to incorporate 45° fibers, and other off-axis fibers, into the weave. Also, in 3-dimensional weaving, as with the stitching method, the stitching material is tightly woven around the composite fibers, leaving no way to join one laminate to another with Z-direction bondline reinforcement.
In the needling method, the fibers of the preform laminate that extend in the X and Y directions are chopped into small pieces by barbed needles. This results in a tangled mass of chopped fibers, including fibers in the Z direction. The drawbacks associated with fiber chopping are obvious. Although fiber chopping does produce some fibers in the Z direction, the fibers in the X and Y directions are cut, and any predefined properties of the preform in the X and Y directions are significantly depleted.